The invention relates to a magnetic bearing arrangement for the rotors of turbo-molecular vacuum pumps wherein the four radial degrees of freedom of a rotor are stabilized by passive magnetic radial bearings and the axial degree of freedom is supported by dry-running mechanical thrust bearings. This axial bearing arrangement is effected in such a manner that the axial position of the rotor is held in such a position that the bearing force of the rotor always remains below a predetermined value.
Such a bearing arrangement is described in the German specification No. 28 25 551 as laid open. In this case, the radial degrees of freedom are fixed by non-regulated magnetic radial bearings. These radial bearings consist of permanent magnetic subassemblies which are mounted, with radially opposite magnetization, on rotating and non-rotating parts. This type of bearing arrangement, however, causes an unstable position of equilibrium in the axial direction. Therefore, if the rotor moves slightly out of the axially unstable position of equilibrium, forces arise which move it further out of the position of equilibrium. These forces increase with the distance of the rotor from the position of equilibrium so that the rotor presses on the thrust bearing arrangement with a multiple of its weight. The German specification No. 28 25 551 describes how these forces are kept below a predetermined value.
Their magnitude is determined by force measuring devices. If these forces exceed a certain value, then a regulating mechanism is set in motion which holds the rotor in the vicinity of the unstable position of equilibrium so that the axial bearing forces do not exceed a predetermined value. This type of mounting is described in the above specification in three different embodiments, which, however, have important disadvantages in practical applications:
(a) The axial position of the rotor is recorded by an electrical position measurement. Contacts are closed when the rotor is in the unstable position of equilibrium within predetermined limits and they are opened as soon as the rotor moves out of this predetermined axial position, to one side or the other. This arrangement has the disadvantage that, at the very high speeds of rotation usual in turbo-molecular pumps, the making of the contacts is very uncertain and accordingly no clear regulating signals can be obtained. In addition the necessary control gearing at the high-vacuum side is very disruptive because it occupies a great deal of room and so, inter alia, sensibly reduces the input conductance of the pump.
(b) The axial position of the rotor is determined by a force measuring device at both sides of the rotor and is regulated by control gearing likewise at both sides of the rotor. This arrangement has the following disadvantage:
Turbo-molecular pumps are generally heated in order to achieve a high final vacuum. As a result of this heating, the housing expands. Since the non-rotating parts of the mechanical thrust bearing are rigidly connected to the housing, the spacing between bearing surface and rotor journal in the thrust bearing is thus increased. If the rotor is bearing against the upper bearing surface of the mechanical thrust bearing, regulation takes place and the expansion is compensated for. On the other hand, if the rotor is bearing against the lower bearing surface of the mechanical thrust bearing, then the upper bearing surfaces moves away from the rotor as a result of the expansion and regulation cannot respond because the predetermined force is not exceeded in this case. Now if, for example as a result of a shock, the rotor jumps to the other side of the mechanical thrust bearing, it may find itself at a large distance from the unstable position of equilibrium and so far outside the predetermined limits, as a result of which inadmissibly high bearing forces may occur. A further disadvantage, as under (a) is the large space requirements of the control gear at the high vacuum side.
(c) In order to avoid these disadvantage of the control gear at the high vacuum side, a further device is proposed in the above specification with which there is no gear at the high vacuum side. In this case, the journal of the lower mechanical thrust bearing experiences the full wear which may lie within the order of magnitude of a few mm. This can be accepted if the device is appropriately designed. At the upper bearing, however, wear means a deviation from the unstable position of equilibrium. This is very critical as the following example shows: With an axial unstable rigidity of 300 n/mm and a predetermined bearing force of 1N, the deflection of the rotor out of the unstable position of equilibrium should not be greater than 1/300 mm. This means that with greater deflection, the life of the rotor journal is very limited.